nexus/physics 132, spring 2014 ta guide—lab 10: analyzing...

NEXUS/Physics 132, Spring 2014 TA GUIDE—Lab 10: Analyzing light spectra and exploring implications for living systems.

TA GUIDE—Lab 10: How can measurements of light spectra provide insights into the nature of matter and the

characteristics of living systems? Spectroscopy—Exploring Emission, Absorption & Evolutionary Adaptation

Introduction:

In this two-week lab, students use a variety of spectroscopes (rudimentary to high-tech) to explore emission and absorption of light. Examining the Hydrogen spectrum, students investigate the Bohr model of the atom—in particular the electron level transitions that result in the Lyman, Balmer, and Paschen series. Students then explore how different combinations of sources and filters affect observed spectra, including everyday filters such as water or sunglasses. Using this accumulated knowledge, students hypothesize which environmental factors may have constrained the evolution of the spectrum that is visible to the human eye and gather evidence to support the plausibility of their hypotheses.

Rather than re-hash all of the student document contents here, please read this TA Guide in parallel with the student document.

In this two-week lab, we will be exploring the interaction of light with matter. Using spectroscopy (also called spectral analysis, spectrometry, or spectrophotometry), we will examine emission and absorption of light by various substances. Spectroscopes (also called spectrometers and spectrophotometers) are measurement tools designed to distinguish different colors of light. The spectrometers we will use in this lab detect the intensity of the light (the power-per-area associated with the light) as a function of the wavelength of the light. These spectrometers have a sensitivity range from 350 nm to 1000 nm, with a spectral resolution of 2 nm. In contrast, the human eye has a sensitivity range from approximately 400 nm (violet) to 700 nm (red), with a spectral resolution ranging from 1 nm (in the green to yellow range) to 10 nm (in the violet and red ranges). Students may have been exposed to spectroscopy data through their previous chemistry and biology courses. To thoroughly understand data, one must know where it came from, its limitations, and its interpretations and potential meanings. With this lab we hope to deepen students' comprehension of the function of spectrometers and thereby enrich their understanding of the data they produce. This will lay a firm foundation for their use of spectroscopy in their broader scientific career.

Developed by: K. Moore, J. Giannini, K. Nordstrom & W. Losert (Univ. of Maryland, College Park)


The lab will consist of three parts: I) exploring the quantized atom; II) exploring emission and absorption; and III) considering the evolutionary adaptation of the visible spectrum of the human eye. The lab report students turn in at the end of the second week should discuss answers to questions posed in the sections below as well as any insights you gain from your explorations and investigations, along with the data supporting those insights.

About the Equipment:

Each group has access to:

a spectrometer and fiber optic cable that can interface with the computer via the USB,

Logger Pro software for the computer,

a small stand with multiple 90°-clamps,

a long pole with table clamp, an assortment of filters/cards,

including Green, Red, Yellow, and UV filters and a diffraction grating,

a variety of hand-held diffraction grating spectrometers,

a classroom set of light sources/lamps: H, Na, Hg, and UV,

small LED light boxes emitting different colors of light (Red, Green, and Blue),

a light ray box (with an incandescent bulb that acts as a white light source),

a meter stick, and an aquarium (fish tank) that can

be filled with water.

BE GENTLE WITH THE EQUIPMENT!! Handle all heavy equipment with care. Watch out for all of the wires and cables.


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Keep all fluids away from the computer and spectrometer. Students may be operating in low light conditions, so they should proceed with caution. The Na and Hg lamps will need to warm-up before they produce a usable spectrum. Masking the light ray box will help reduce glare from reflections. Students may find it helpful to use the fiber optic sensor in conjunction with a fixed holder/mount.

A Short Introduction to Light/Quantization:

Useful info for TA:

Visible portion of EM spectrum (sensed by the human eye) is ~ 400nm - 700nm. Speed of light in a vacuum c = 3.0·108 m/s. Frequency, f, and wavelength, λ, are related with the wave speed, v, according to: v = fλ. The energy, E, carried by these waves is proportional to their frequency: E=hf, where the proportionality constant, h, is Planck's constant (h = 6.63·10-34 kg·m2/s).

Part I: Exploring the Quantized Atom

Perhaps the most 'illuminating' model of the quantization of light is the Bohr model of the hydrogen (H) atom. In this model, the proton nucleus of the hydrogen atom is orbited by the single electron at fixed orbital radii. When the atom is excited (absorbs energy), the electron transitions from the ground state (lowest energy level) to an excited state. The electron will then transition back to a lower energy state, emitting this energy difference between levels as a photon of light. The Bohr model is a good model for explaining why only certain frequencies/wavelengths of light are absorbed or emitted by an atom. It also explains the Rydberg formula for the emission lines of the hydrogen spectrum—this formula was well-established experimentally as early as 1888, but had no theoretical explanation until the Bohr model was introduced in 1913 (25 years later!). TA Note: This idea—that science can be done even when an answer (or the theoretical underpinnings of an answer) are not known—is something with which students can be deeply uncomfortable. School science rarely emphasizes this aspect of the nature of scientific inquiry. Lecture presentations of information, often disconnected from the historical contexts within which science advancements were made, further undermines students' ability to form an accurate, multidimensional, and complex understanding of the nature of science and scientific experimentation.



Some of the energy level transitions within the hydrogen atom have been given specific names. All transitions to/from the ground state (lowest energy level, n = 1) are part of the Lyman series. All transitions to/from the 1st excited stated (n = 2) are part of the Balmer series. All transitions to/from the 2nd excited state (n = 3) are part of the Paschen series. Other transitions also have formal names, but the Lyman, Balmer, and Paschen are the most commonly encountered. Each level (each n) has an associated energy given by: En=(-13.606 eV)/(n2). (Recall that 1 Joule = 6.2415·1018 eV.)

Below are three charts showing the energy levels for the Bohr model of the hydrogen atom and the associated energy level transitions. How are these similar? How are they different? Are any of the charts communicating the information in more helpful/clear ways? Are any of the charts misleading in the communication of information? Which chart does your group like best?

Chart 1 Chart 2 Chart 3

TA Responses:

We would like to know which of these series is observed in the hydrogen emission spectrum. Using the hydrogen lamp and the spectrometer with fiber optic cable, collect data for the emission spectrum of hydrogen. To collect the data with Logger Pro, open the My Documents folder on the Desktop and double-click on “Spectrometer.cmbl.”


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http://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=gfQTtlZHipRPkM&tbnid=5BZ-VUTOR8qYRM:&ved=&url=http://people.rit.edu/agysps/courses/320PrOpt/worksheets/BuildAtom.htm&ei=bcZqUc64OOLH0wGK3oCICw&bvm=bv.45175338,d.dmQ&psig=AFQjCNHQAnAeJiq2gLu9GXKG45TV4GfszQ&ust=1366038510230485

http://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=foH_vJI6sV2jWM&tbnid=-CUew1t1hTo5qM:&ved=&url=http://astro.unl.edu/naap/hydrogen/transitions.html&ei=bcZqUc64OOLH0wGK3oCICw&bvm=bv.45175338,d.dmQ&psig=AFQjCNHQAnAeJiq2gLu9GXKG45TV4GfszQ&ust=1366038510230485


Clicking on the green "Collect" button will enable students to gather information. Direct the end of the fiber optic cable toward the light source (the pin-hole on the end of the cable is where the light enters—it is sensitive to direction and distance from source, so keep these considerations in mind). To keep the data, click "Stop." When students wish to gather another data set, click "Collect"—they can overwrite the previous data set OR they can keep the old data and display the new data as well as the old data (choose "Store Latest Run"). To get rid of a stored data set, choose Data → Delete Data Set and select the appropriate set for deletion. When they have a good, clean, clear emission spectrum for the hydrogen lamp, students should print out their emission spectrum graph (of power vs. wavelength).

Which wavelengths are present in the hydrogen emission spectrum? Do these transitions represent the Lyman, Balmer, or Paschen series? Students should justify their claim by comparing their data to the Bohr model.

As noted earlier, the Bohr model gave a theoretical reasoning for a long-standing experimental result. The Rydberg formula for the hydrogen emission spectrum is written below. Using their data, students should find the Rydberg constant, R. What is the range of uncertainty for the value of R?

Part II: Exploring Emission and Absorption

Now let’s investigate emission of light by different sources and the ways in which absorption affects the recorded spectra. Students should use the hand-held diffraction grating spectrometers (they have three types of these) and the diffraction grating card to get a qualitative sense of the spectra produced by different sources (including the sun light from the



window and the fluorescent lights in the ceiling). For the best quality data, students should use the spectrometer with fiber optic cable, which they may wish to attach to a clamp-and-stand set-up. Students should try different combinations of sources and filters. Their task in this portion of the lab is to develop an understanding of what spectra are produced by different sources and how these spectra compare and contrast. Additionally, they should develop an understanding of how absorption (by glass, water, various filters, solid-colored surfaces, etc.) affects the transmitted spectrum. There is a lot of different equipment involved in these investigations, so students should rotate themselves to different tables (rather than shifting the equipment between tables). Students should remember to take notes on the observations and insights they gain from their investigations. To aid in this exploration, students might consider the following questions (these are suggested, but not mandated):

How do the various filters change the spectrum produced by the hydrogen lamp? What does a ‘red’ filter do? How are the spectra produced by the sodium (Na) or mercury (Hg) lamps similar to

and different from that produced by the hydrogen (H) lamp? How is the sun’s spectrum different from or similar to those produced by the lamps (H,

Na, and Hg)? Is the sun’s spectrum still ‘quantized’? Why or why not? Is the sun’s spectrum the same outside the window as it is when the sun has travelled

through the window’s glass? (Does the glass absorb any light? If so, which frequencies or bands of frequencies are most affected?)

What do you see when you look at an incandescent (heated filament) source, such as the bulb in the light ray box? How does this spectrum compare to the sun’s?

How do the UV (ultra-violet) and LED light sources compare to the solar spectrum or to the incandescent spectrum? How do the filters change the spectra observed from these sources?

Does water act as a filter? If so, which frequencies or bands of frequencies are most affected?

What factors affect the observed color of different reflective surfaces (like your t-shirts or your notebook covers)?

What effect do combinations of filters have on observed spectra? What frequencies or bands of frequencies are affected by using your sunglasses as a

filter? Are all sunglasses the same? How does the distance of the fiber optic sensor from the source affect the observed

spectra?

Before they move on to Part III, students should consider what mechanisms inside the spectrometer (with the fiber optic cable) enable it to detect different wavelengths of light. What are the physical mechanisms within this ‘black box’ measurement tool? Students should discuss this as part of their lab report.



Part III: Considering the Evolutionary Adaptation of the Visible Spectrum of the Human Eye

The mechanisms by which living creatures ‘see’ the world around them vary significantly across species. Even more remarkably, this diversity of evolutionary adaptations exists even though all living things were exposed to the same ultimate source: the sun. The solar spectrum is, in fact, very different from the spectrum to which the human eye is sensitive (often called the ‘Visible’ spectrum). Many insects and some birds are sensitive to wavelengths, particularly in the ultraviolet, that are completely invisible to the human eye; thus, the visible spectrum of a bee or a bird can be quite different from that of a human.1 Bees, birds, turtles, lizards, many fish and some rodents have UV receptors in their retinas. These animals can see the UV patterns found on flowers and other wildlife that are otherwise invisible to the human eye.2 The schlieren photograph (previous page) shows the zones of a fish lens that focus different spectral ranges (red on the outside edge, toward UV at the center). Some animals, such as reptiles (e.g. snakes), have IR (infra-red) sensitivity.

Why have humans developed eyes sensitive only to the Visible spectrum (~400 nm – 700 nm)? Why are the UV and IR bands, present in the solar spectrum and in the visible spectra of other living creatures, absent from the visible spectrum of the human eye? Your students' task is to develop plausible hypotheses for the evolutionary adaptations resulting in the absence of UV and IR sensitivity in the human eye and to collect spectral data to support the plausibility of their hypotheses. The following two charts may (or may not!) help them get started in thinking about plausible hypotheses.

1 From: http://www.webexhibits.org/causesofcolor/17.html2 From Wikipedia: http://en.wikipedia.org/wiki/Color_vision#In_other_animal_species



Absorption Coefficient for Water (H2O) Solar Spectrum: Outside Atmosphere & At Sea Level, w/ H2O Absorp.

TA Responses:

For their report:

The lab write-up should include careful discussions of (a) their insights and investigations for Parts I and II, (b) discussions of any data that support these insights, (c) their investigations into the evolutionary adaptation of the visible spectrum of the human eye (Part III), and (d) their comparison of their work with the work of other groups, as well as their critique of their experiment and conclusions. If they want to include hand-drawn elements (such as ray diagrams), please encourage them to do so. Students should also consider what is going on inside the spectrometer connected to the USB/computer and discuss the physical mechanisms inside this 'black box.'



Approximate timing:Week 1: (following Spectroscopy Recitation)

Introduction .................................................. 15 min.

Part I Investigation ……............................. 30 min.

Part II Investigation ..................................... 40 min.

Start Part III Investigation .......................... 25 min.

Week 2:

Finish Part III Investigation ….................. 40 min.

Create Posters/Presentations ..................... 20 min.

Presentations/Discussion ........................... 30 min.

Finalize Report ............................................. 20 min.

Discussion/Comment Suggestions for Week 1:

Introduction to provide to students: (N.B.: This is an exploratory lab, so please DO NOT share the skill goals with students until the very end of the second week of this lab!!)

Groups of three or four students—performing in the Community Lab roles. Ask the students who has worked with spectroscopes/spectrometers before (this

should be everyone!). Get them to share the contexts in which they have used spectrometers and the specific application/use of the spectrometer. Ask if any of the students know how a spectroscope/spectrometer operates (this should be almost no one!).

There is not one set of all lamps for every table. Instead, the equipment will STAY at a station/table and the STUDENTS will move as a lab group from station to station to do their investigations.

Equipment notes: Treat the fiber optic cables GENTLY. Lamps will heat up, so be careful with the hot equipment, and turn the lamps off when not in use (except for the Hg and Na spectral lamps—leave these on, as they only give a proper spectrum if they are all warmed-up). Clamps should not be over-tightened. Optical elements should be aligned properly. Handle all heavy equipment with care. Watch out for all of the wires and cables. Keep all fluids away from the computer and spectrometer. Students may be operating in low light conditions, so they should proceed with caution. The Na and Hg lamps will need to warm-up before they produce a usable spectrum. Masking the light ray box will help reduce glare from reflections. Students may find it helpful to use the fiber optic sensor in conjunction with a fixed holder/mount. All equipment should be turned off when the lab is done, and all lab documents should be sorted and stacked at the front of the room.

Encourage your students to wear bright, solid colors. Encourage them to bring their sunglasses to class. These things can be used as light sources (for reflections) and as filters. If they don't have these ready for the first week, they can bring them to the second week of lab.



Remind students that, when students leave the lab room today, EACH student in the group should have an electronic copy of ALL of the group's data (via email or flash-drive).

Remind students that they are ALL expected to master the skills, so they should take turns and help each other out. Taking notes may not be a bad idea, either.

Inform students that a lab report will be due at the end of the lab next week—but it is a good idea to write as much as they can of the report this week, so that they don’t forget what they did!

While they are working:

Make sure that each group is getting a chance to work with the hydrogen lamp to collect the emission spectrum they need for Part I. Once each group has a spectrum, they can go to a new table to enable the next group to gather their data. Part I should be done before Part II, but if students want to use the Logger Pro and spectrometer to gather some emission spectra data from other light sources while they wait their turn with the hydrogen lamp, that's fine.

Summation at the end of the lab period:

o Ask the students what they found most difficult/challenging about either Part I or Part II (most groups will not reach Part III in the first week—the same equipment will be available in subsequent weeks).

o Remind students to neatly put their lab documents back and to ensure that all light sources are turned off.

o Remind students to save their work (data, Excel files, Word documents, etc.).o In the second week, students will finish Part II, do Part III, and finalize their

presentations and reports.

Discussion/Comment Suggestions for Week 2:

Introduction to provide to students:

Remind the students of the topical content and the goals of this two-week lab. Ask the students to recall what they did in the first week of this lab. Discuss that today's tasks are to finish Parts II and III of the lab and to compile

their information into a presentation (for their peers) and a lab report (for their TA).

Remind students that, when they are finished with the lab, all equipment should be turned off and straightened up.

Remind students that they are ALL expected to master the skills, so they should take turns and help each other out. Taking notes may not be a bad idea, either.

Inform students that the lab report is due at the end of the period. There will certainly be time for poster presentations, so students should think

carefully about how they can present their investigation, their plausible hypotheses, and their results to their peers.

Summation at the end of the lab period:



o Discuss the Skill Goals, clarifying any remaining confusion about physics concepts.

o Discuss the challenges and considerations with the class, if any have not yet been addressed.

o Collect the lab reports.

Physics Skill Goals: Challenges/Considerations:Understand and employ basic light vocabulary: quanta, quantized, wavelength, frequency, wave speed, emission, absorption, spectrum, energy level, photon, etc.

Why is light quantized? How is a photon a quantum of energy? How are wavelength, frequency, and wave speed related? How are wavelength, frequency, and wave speed related to energy? How are emission and absorption similar/different? How can a measured spectrum give us information about the spacing of the energy levels in a system?

Compare and contrast multiple representations of energy level transitions in the Bohr model of the hydrogen atom.

How are these similar? How are they different? Are any of the representations communicating the information in more helpful/clear ways? Are any of the representations misleading in the communication of information?

Deepen comprehension of the nature of scientific investigation.

How is the inquiry-in-the-absence-of-theory mode represented in the various scientific disciplines? What is the interplay between theory and experiment in scientific investigation? Must one always precede the other?

Determine whether the visible light emitted by a hydrogen lamp belongs to the Lyman, Balmer, or Paschen series.

How can the energy level changes for the Lyman, Balmer, or Paschen series be converted into wavelength or frequency information? How can we compare these theoretical values to the measured spectrum from the hydrogen lamp?

Compare the measured spectrum for a hydrogen lamp to the Rydberg formula to determine the Rydberg constant.

How do we perform the necessary mathematical manipulations to compare our experimental data to the experimental mathematical models constructed by other scientists? Should we expect our results to agree? Why?/Why not?

Measure emission spectra for a variety of light sources in combination with a variety of filters.

What information is being displayed by the Logger Pro as it interprets the spectrometer data? How certain are these measurements? Is the uncertainty dependent on the wavelength of the light? What aspects of the physical measurement (e.g., distance from the source, ambient lighting conditions, directionality of the fiber optic sensor, etc.) affect the recorded data?



Physics Skill Goals: Challenges/Considerations:Compare and contrast spectral measurements.

How do different light sources compare? What is the role of a filter? Does water act as a filter? (Et al., see questions in Part II.)

Compare and contrast available data to construct a plausible hypothesis.

How do we generate hypotheses? What does it mean for a hypothesis to be 'plausible'? How do we determine what data does or may support the plausibility of the hypothesis? Does all collected data need to be used to generate a hypothesis or to support the plausibility of the hypothesis?
